Artificial hip joints, which are known as hip joint endoprostheses, are often used to treat patients whose hip joints show excessive signs of wear and tear. Other indications are conditions that are known as coxa vara and coxa valga. Coxa vara, that is to say an “outward curvature of the hip”, describes the condition where the angle known as the CCD angle (centrum-collum-diaphyseal angle), that is the angle formed between the femoral neck and the shaft of the femur bone, is less than 120°. When the CCD angle is about 125°, this is referred to as a “normal hip”, or coxa norma. The opposite condition of coxa vara is coxa valga, which often occurs in young children and newborns and represents an increase in the CCD angle beyond 135°. The deviations from the norm result from growth—coxa valga situations are normal in newborns—and pathological changes. For example, coxa vara often develops in elderly persons. However, there is also wide variation in the geometries of the proximal femoral area based on national differences.
In endoprosthetic hip joint replacement different medial contours of the femoral canal, different angles of the femoral neck, and associated therewith also the resulting positions of the head centre point of the hip joint endoprostheses are of particular importance. In order to enable appropriate treatment of these three above-described classes of coxa valga, coxa norma and coxa vara, secure fixation of the implant in the femoral canal needs to be ensured. In this regard and depending on how the implant is fixed in the femoral canal, medial contact of the prosthesis stem with the cortical bone of the femur is very important. Therefore, an implant system is always optimized for one of the three groups mentioned and cannot be used at all or only under exceptional conditions in caring for the other groups. Furthermore, what is also characteristic of the three groups mentioned is the position of the rotational centre of the joint. Compared with a standard femur, a valgus femur has rather a small offset and a varus femur has rather a large offset. This range and in particular the interplay between specific medial contact and position of the head centre point cannot be covered with the usual systems.
One problem which exists with the implantation of the different hip joint endoprostheses is that extensive instrumentations are required depending on the patient's pathological situation.